Apparatus for removing dissolved impurities from liquid alkali metals



1958 E. F. BATUTIS ETAL 2,356,702

APPARATUS FOR REMOVING DISSOLVED IMPURITIES FROM LIQUID ALKALI METALS Filed 001:. 4, 1955 2 Sheets-Sheet 1 COOLING FLUID IN 6 MEANS FOR U RETYR-ACTI'N'G COLD FINGER 2 GATE VALVE 0R NoK W K 51%, 1'. MW

INVENTORS Dec; 30, 1958 E. F. BATUTIS ET AL 2,866,702

APPARATUS FOR REMOVING DISSOLVED IMPURITIES FROM LIQUID ALKALI METALS Filed Oct. 4, 1955 2 Sheets-Sheet 2 QTG :2 GATE VALVE IN V EN TORS BY an,

APPARATUS FOR REMOVING DISSOLVED IM- PURITIES FROM LIQUID ALKALI METALS Edward F. Batutis, Zelienople, Pa., and Shelby L. Walters, Pensacola, F la., assignors, by mesne assignments, to the United States of America Application October 4, 1955, Serial No. 538,495

I 1 Claim. (Cl. 75-66) 1 This invention relates to a method and apparatus for removing dissolved impurities from liquid sodium or sodium-potassium alloys. In particular, it relates to a new and improved method and apparatus for removing dissolved sodium or potassium oxide from liquid sodium or sodium-potassium alloys.

Liquid metals such as sodium and sodium-potassium alloys have become increasingly important in recent years as valuable heat transfer mediums for high temperature reactions. One problem which has seriously impeded their use has been the high corrosion rate of systems in which such liquid metals are employed. This high rate of corrosion was later found to be due to dissolved metal oxides in the liquid metals themselves. It was therefore necessary to find some method of removing these oxides in an economical and practical manner.

It is an object of this invention to provide a simple and efiicient method for removing dissolved impurities from liquid sodium and sodium-potassium alloys.

Another object is to provide a new and improved method for removing dissolved sodium oxide from liquid sodium metals.

A further object is to provide a new and improved apparatus for removing metal oxides and other soluble impurities from liquid sodium or sodium-potassium alloys (commonly referred to as NaK).

Other objects will become apparent throughout the following specification and appended claims.

This new and improved method and apparatus for removing dissolved impurities from liquid metals will be more fully described hereinafter and the novelty thereof particularly pointed out and distinctly claimed.

This invention is based upon the discovery that dissolved impurities such as sodium and potassium oxides can be rapidly and efficiently removed from liquid sodium, potassium, or sodium-potassium alloys by inserting a cold finger into the line of a circulating liquid metal system. Such a cold finger, which may be a solid rod provided with a cooling coil or a hollow tube through which a suitable coolant is circulated, produces a cool surface at the junction of the cold finger with the liquid metal and causes impurities dissolved in the liquid metal to crystallize out and collect thereon. The crystals of dissolved oxides cling to the cool surface of the cold finger and can be removed by slowly raising the same.

In the accompanying drawings to be taken as part of this specification there are shown two preferred embodiments of this invention. Fig. 1 is a diagrammatic view of a cold finger inserted through a gate valve into the line of a circulating liquid metal system. In this view the cold finger is a hollow tube provided with an inlet and outlet for circulating a coolant such as water or a gas like nitrogen. Fig. 2 is a diagrammatic view of a similar apparatus in which the cold finger is a solid rod provided with a cooling coil.

Referring now to Fig. l by numerals of reference,

there is shown a container 1 through which liquid sodiunr orLfNaK is continuously circulated by means of a pump 2,866,702 Patented Dec. 30, 1958 (not shown). The container '1 which may be a supply tank or stainless steel pipe is fitted with a gate valve 2 for easy access. Through the gate valve is inserted a hollow tube 3 made of metal or glass and equipped with suitable means 4 for raising and lowering said tube into the liquid metal. An inlet tube 5, also made of metal or glass, for introducing a coolant passes down the center of the entire length of tube 3. An outlet 6 is provided to allow the coolant to circulate through tube 3. A cold surface 7 is thus provided at the junction of tube 3 with the liquid metal where any dissolved metal oxides contained in the liquid metal crystallize out.

In operation the apparatus shown in Fig. 1 is assembled on the gate valve 2 with the cold finger 3 in its uppermost position. The gate valve is opened and the cold finger lowered to a point below the surface of the liquid metal 1. The cooling system 5 and 6 is maintained at a flow rate such that the cold finger surface exposed to the liquid metal 1 is cooler than any other surface in contact with the liquid metal. After a predetermined interval, the cold finger 3 is raised slowly to expose a new surface for further purification of dissolved oxides until it is no longer in contact with the liquid metal 1 and then with a smooth even motion raised above the entry port at which time the gate valve is closed. The oxides accumulated on the cold finger at point 7 are discarded. This process is repeated until a desired level of purity is reached in the liquid metal system.

In Fig. 2 of the attached drawings, there is shown a modification of the type of cold finger employed in this method in which 11 is a container or pipe through which the liquid metal is circulated. This container is equipped with a gate valve 12 through which is inserted a solid rod 13 said rod having suitable means 14 for raising and lowering it into contact with the liquid metal. Around the exposed end of rod 13 is wrapped a cooling coil 15 made of metal or glass through which a coolant such as water or a gas enters at inlet 16 and leaves at outlet 17. The dissolved oxides contained in the liquid metal are collected at the junction 18 of the rod 13 with the liquid metal.

The apparatus shown in Fig. 2 operates substantially the same as that of Fig. l with the exception that the cold finger is a solid rod surrounded by an external cooling coil and the rod is cooled by conduction.

It has been found in the operation of liquid metal heat transfer systems that the liquid metal will not corrode the system appreciably if the concentration of oxygen in the form of alkali metal oxides in the system is maintained at a concentration less than about .01 to .02 wt. percent. At higher oxide concentration, it has been found that considerable corrosion results and that the oxides present in the liquid metal may precipitate and cause severe damage to the system.

The solubility of alkali metal oxides in liquid sodium and NaK alloys was found to be dependent upon the temperature at which the liquid metal system is maintained. In the table below there is presented data showing the weight percent of oxygen found in liquid sodium and a 78% K-22% Na alloy at various temperatures.

From this table it is apparent that the oxygen content of both liquid sodium and NaK alloys increases rapidly at ternperatl l'fi in excess of 600 F. This increase in oxygen content is considerably more rapid in the case of liquid sodium than when sodium potassium alloys are used. Since most liquid metal systems are operated at 0 E t qh iws t a Xi4 as pn i a e ip s sm- W In several egtperiments which were carried out using the type of cold fingers herein described, the oxygen sa e o th i u s a sssd w s substan r clueed. By repeated operation of the cold finger, the oxygen content of the liquid can be reduced to below .01 wt. percent at which point damage due to corrosion is negligible. In these experiments, the cold finger was maintained at or below the temperature at which the oxides are appreciably soluble. "It should :be noted that if itisdesired to redissolve the accumulated oirides, this may be accomplished by heating the Lcold finger.

Although two preferred embodiments of this ihyention have been described in detail, it will be apparent to tlio'se skilled in the liquid metal art that otherlyariatio'ns are possible and that within the scope of theappe'nd'ed'claims this invention may be practiced otherwiseihan as specifically Having to claim and secure byhlinited states patentis:

An apparatus for removing soluble impurities from liquid alkali metals in a circulating system comprising a container having an entry port therein, a solid metal 7 us dfiSCI'lbQd our ipyention fully and com: p s ,rs t s b he Pa en laws ha we ,Qesire 4 rod extending through said port into the circulating liquid metal in said container, means for cooling the end of said rod remaining outside said container and operable to cool the portion of the rod extending into said container by conduction whereby said impurities are caused to precipitate on said rod at the surface of said liquid metal, and means for lowering the end of said rod into said liquid metal and slowly Withdrawing said rod from said liquid metal whereby a new portion of said rod is continuously exposed at the surface of said liquid metal for precipitation of said impurities.

References Cited in the file of this patent UNITED STATES PATENTS 1,361,853 Hazel Dec. 14, 1920 1,542,842 Schenck June 23, 1925 2,239,371 Osborn et al. Apr. 22, 1941 2,248,909 Russell July 8, 1941 2,271,023 Nelson Jan. 27, 1942 2,615,706 Davey --.i--- OH,- 28, 1952 2,621,120 Pedersen et al. Dec. 9, v 1 9 5 2 2,745, @B u m n a a a Ma 15 9 6 OTHER REFERENCES Scientific Instruments and Laboratory Suppliesfl Central Scientific Co., Catalog 1-7150, 19 50, page'21 3.

Mechanical Engineering, 'vol. 75, June 1953, pages 72-47 

